This invention relates to a process for the hydrothermal decomposition of aluminum silicates and alkali aluminum silicates with aqueous sodium hydroxide solution under pressure and at elevated temperatures in the presence of a large excess of silicates which are soluble under these hydrothermal conditions.
Both mineral aluminum silicates and alkali aluminum silicates and also the technically preferred synthetic aluminum silicates and alkali aluminum silicates are sparingly soluble in aqueous sodium hydroxide solution, even under pressure and at elevated temperatures. Accordingly, compounds such as these are for the most part commercially produced by hydrothermal techniques. To this end, an aqueous silicate solution containing free alkali and an aluminate solution likewise containing free alkali are combined, resulting immediately in the precipitation of a corresponding alkali aluminum silicate at temperatures above room temperature. Even when this suspension containing excess alkali is subsequently subjected to hydrothermal aftertreatment at temperatures of the order of 200.degree. C. and higher, for example for the purpose of crystallization or recrystallization, the solid is not dissolved to any significant extent. The sum total of dissolved silicate and dissolved aluminate always amounts to distinctly less than 1% by weight. It is precisely this stability of aluminum silicates and alkali aluminum silicates with respect to alkaline solutions which is commercially utilized for filtering aqueous alkali silicate solutions, despite their occasionally very high alkali content, at temperatures of the order of 100.degree. C. and higher using fixed-bed filters with filter aids based on aluminum silicate. On completion of this filtering operation, the so-called filter sludge of filter aids and the insoluble constituents of the solution to be filtered has hitherto been discarded, i.e. pretreated and/or dumped at considerable expense. Similarly, industrial waste products based on alkali aluminum silicate, of the type which can accumulate for example through defective crystallization or even as operationally unavoidable secondary products in the synthesis of molecular sieves, have hitherto always had to be dumped, generally through lack of any commercial value.